Does it make any sense to run
umount during a clean shutdown? I assume that
umount will write to disk any cached data.
The only odd case I can think of is when you have some loop device (like a LUKS container) on the top of your real disk.
Umount might try to unmount the partition first. And it might not be able to unmount a partition until the loop device is unmounted. But would it sync the data anyway in this case?